1. Field of the Invention
The present invention relates to an on-board tracking antenna system in vehicles such as motorcars and the like, which is adapted to automatically direct an antenna beam to a broadcasting satellite and a communications satellite.
2. Description of the Related Art
Recently, the demand for satellite communications and satellite broadcasting has been increased due to their advantages such as wide area coverage and excellent picture quality. Further, satellite broadcasting and communications have been more and more utilized not only in houses but also in moving vehicles.
Such satellite communication or broadcasting re-quires that the antenna on the vehicle is always directed toward the satellite in order to maintain a desired sensitivity of reception as the vehicle moves. Since the vehicle will freely change its direction while in motion, the antenna must be rotated to all the azimuth directions, that is, through 360 degrees.
A prior art tracking antenna system for controlling the direction at the antenna to be towards the communications or broadcasting satellite is shown in FIG. 11. The tracking antenna system comprises a turntable 10 and a planar antenna 12 with an elevation drive device 14 mounted on the turntable 10. The turntable 10 itself is rotationally driven by an azimuth drive device 16. The tracking antenna can be mounted on the roof of a motorcar or the like and can always be directed towards the satellite through the actuations of the elevation and azimuth drive devices 14 and 16. In Japan, an elevation angle is frequently set to be in the range between about .+-.10 degrees and .+-.20 degrees relative to the direction of the satellite.
In order to track the satellite, the tracking antenna system must detect the satellite direction at all times. To this end, the prior art has always detected a signal level by continually changing the direction of the antenna gradually such that the maximum signal level will be obtained. In order to associate the automated tracking operation more accurately with the movement of the motorcar, a gyrocompass or geomagnetic sensor has been mounted in the motorcar to sense the orientation of the moving motorcar which is in turn combined with the information of the satellite to thus track the satellite.
However, the prior art tracking antenna necessarily requires an increased height to obtain a sufficient gain.
More particularly, a small-sized antenna may be an array antenna comprising a plurality of micro strip or slot antenna elements. On the other hand, the gain in such an antenna should be equal to or higher than about 30 dBi as in the reception of satellite broadcasting. Thus, such an antenna is required to have 300 to 500 micro strip or slot antenna elements which are arranged in the desired pattern. This results in an increase in the size of the antenna 12, which is a difficult problem to overcome.
When the beam direction of the antenna is continually changed to track the satellite, any unnecessary motion of the antenna may adversely affect the reception.
The present invention is directed to improve the detection of the satellite in the aforementioned tracking antenna system.